Photoelectric apparatus



y 2, 1940- K. T. BAiNBRlDGE PHOTOELECTRIC APPARATUS Filed Aug. 28, 1929 Inventor: Kenneth T Bainbridge,

His Attorney.

. Patented July 2, 1940 UNITED STATES rArsT FFiE 2,206,713 PHOTOELECTRIC APPARATUS Kenneth T. Bainbridge, New York, N. Y., assignor to General Electric Company, a corporation of New York Application August 28, 1929, Serial. No. 389,066

8 Claims.

0 may consist of those devices which depend upon ionization between the electrodes in which case, the envelope contains a substantial, amount of gas. Photoelectric tubes of the highly evacuated type are utilized in systems Where uniformity of operation and rapidity of response arenecessary as in the case of rapid variations of light but have the disadvantage of producing photocurrents of small magnitude calling for one or more stages ofamplifying accessories in: order to derive a current of practical amount. On the other hand, while the gas-filled tubes give photocurrents of a larger order than those of thepure electron discharge type, hence necessitating less amplification if any, they have the disadvantage of a nonlinear response when considered in terms of photocurrent per unit of light intensity and in addition may ofier a time lag when dealing with light of an undulating character as in television.

Moreover, in case the gaseousphotoelectric devices are operated at potentials which produce substantial ionization, the light sensitive surface may be destroyed by positive ion. bombardment. An object of-the present invention is to provide a photoelectric tube whichwill combine the unif formity and rapidity of response peculiar to the pure electron discharge type with the desirable feature of producing a photocurrent of substantial magnitude as found only in the prior artgasfilled tubes without introducing the disadvantages usually inherent in the latter. The object is thus to combine the desirable features of both types of tube, at the same time eliminating the undesirable properties. In carrying out the invention, I propose to utilize anenvelope devoid of gas andto increase thecurrent-carrying capacity of the device by the use of secondary emission phenomena, The invention will be better understood when reference is made to the following description and the accompanying drawing. The figure represents a perspective View in elevation of the improved photoelectric tube, connected in a circuit for explanatory purposes.

In the drawing, numeral l designates a highly evacuated envelope terminated by a reentrant stem and press 2. In the press there are fused three rigid wires leading to a plurality of electrodes 3, t and 5. These electrodes are shown as rectangular in shape, extending in the same direction and arranged in vertical planes when referred to the tube position as shown, the two outer members being angularly disposed with respect to oneanother in the horizontal plane and having the central electrode arranged preferably equidistantly between the outer members.

The electrodes 3 and 5, which may be termed for convenience, primary cathode and secondary cathode respectively, are constituted of a solid member while the central electrode, which may be ccnsidereda collector or anode, takes on the general form ofa grid with finewires and a f I relatively wide mesh. The cathodes 3 and 5 are each rendered light-sensitive although as will be explained hereinafter, it is necessary for satisfactory operation to provide a photoelectric surface only on the more negative cathode, i. e. elec-. trode 3. In the preferred arrangement the cathodes are cbnstituted of a base metal such as nickel or copper which may be silver-plated preferably apart from the envelope in which they are to be employed by any well known process e. g. by electro-plating, chemical deposition, or electrical sputtering as disclosed and claimed in the Buggles Patent #l,568,694. The silver coating is oxidized, preparatory to receiving the light-sensitive material, .by immersing the plates in an oxidizing atmosphere preferably confined in envelope I or, a separate bulb and glowing the electrodes by applying high voltage thereto from a spark coil. fIhe oxidation may also be produced by applying a high voltage direct current derived from a suitable rectifier set. In. either case, the depth of the oxide layer may be controlled by the duration and intensity of such treatment. When the desired degree. of oxidation has been effected, the plates, if not already placed in position, may be mounted in the envelope I together with the grid-like anode i. In. order to sen: sitize the silver or other metallic oxide surface on the cathodescaesium or other suitable alkali vapor is introduced into the envelope either by distillationfrom a side tube or from pellets heated oxide which does not combine in a substantial degree with caesium, the anode surface remains practically insensitive to light. Theexcess alkali material preferably is removed from the envelope in any suitable manner, for example, by distillation and evacuation of the excess vapor to the manifold, in order to lend to the device uniformity of operation as disclosed and claimed in my prior application Serial No. 244,533 which has matured into Patent No. 1,901,577, granted March 14, 1933. The primary cathode 3 and the anode 4 may be connected to a source of direct current 6, the member 4 being positive while the secondary cathode 5 is connected to an intermediate terminal l in the potential source. This terminal preferably should be at such a position that the potential difference between electrodes 3 and 4 is of the order of several times the potential difference between electrodes 4 and 5, for example, ten times. It will be noted that the cathodes 3 and 5 are both negative with respect to the anode or collector 4. When a light beam from an eX- ternal source indicated generally by the cross '8 impinges on the primary cathode 3, the latter hundred volts.

r1 between electrodes 3 and electrode 5.

, be connected between electrodes 4 and trostatic field between this electrode and anode 4 to a velocity equivalent to thetotal voltage of battery 6 which may be of the order of several A substantial part of these electrons pass through the mesh of the anode 4 and impinge on secondary cathode 5 with an impact sufficiently great to give rise to secondary electrons in considerable numbers at the surface of These electrons are propelled to- Ward the anode or collector 4 under the influence of the electrostatic field between the secondary cathode and the more positive collector. this field is in the opposite direction to the field 4, it is apparent that its retarding effect on the primary electrons travelling from the cathode 3 is small because the potential difference between electrodes 3 and 4 is many times the potential difference of the decelerating field between electrodes 4 and 5. It is evident that the potentials assigned to the various electrodes may be such as to produce a large preponderation of the secondary electrons at the surface of the secondary cathode over the number of primary electrons initiated at the primary cathode by light activation and that this electron preponderation may be utilized in an external circuit by way of photocurrents of au mented amplitude. This circuit normally would 5 at the place indicated by the pair of open terminals. In each of the electrode leads, there may be inserted current measuring devices 9, If! and H to determine wh n the greatest sensitivity of operation is reached in terms of magnitude of photocurrent as the inter-electrode voltages are regulated. It is obvious that the ammeter 94 will register the total primary electron current leaving the surface of cathode 3, the ammeter H] indicating the combined secondary photocurrents and aportion of the primary photocurrents depending upon the fineness of the grid and other factors. The ammeter II will read the difference between the remainder of the primary electron current which reaches the surface 5 and the combined secondary electron current and photocurrent leaving this surface. In order further to enhance the primary and secondary electron flow, the plates 3 and 5 may be arranged angularly with respect to one another thereby to obtain a multiple reflection effect of the light beam loses electrons which are accelerated in the elec- While the cathode 3 and which augment to a certain extent the number of electrons leaving the oathode 5 and travelling toward anode 4.

From the foregoing it is evident that I have disclosed a novel form of photoelectric tube which operates on a pure electron basis, hence offers all the advantages of this type of device, at the same time, is capable of producing photocurrents of several times the amplitude of those generated in the prior art tubes which depend solely on primary electron flow. The increased order of photocurrent amplitude may, by proper tube design and regulation of the applied voltages, come within the range of currents heretofore reached solely in gas tubes, the advantage being obtained without introducing the deleterious effects which usually gas type. Inasmuch as the production of the secondary electrons at the surface of secondary cathode 5 is dependent upon the impingement therewith of the primary electrons produced at cathode 3 and further since the primary electrons are controllable by the light beam,it is apparent that the secondary' photocurrents are also under the positive control of the light.

It will occur to those skilled in the art that the secondary cathode need not have a highly lightsensitive surface but may consist simply of ordinary metal, in which case the increase in current due to multiple refiectionwould not be quite so pronounced, although under certain conditions as when the secondary cathode has high reflectivity, the output current may actually be greater than in a case where this cathode is provided with a surface of relatively high light sensitivity. It is apparent that in the case of a cathode having a specular non-photosensitive surface, most of the light incident thereon would be reflected without the production of photoelectrons at this accompany tubes of the surface and the reflected light would give rise to H [sensitive substance, hence materials which are particularly well suited for emitting secondary electrons may be employed, such as barium oxide (BaO) or metallic barium (Ba) applied as a coat to the foundation metal. Accordingly, my invention is not to be'limited to any particular class of materials for the secondary cathode member 5 but is sumciently broad to cover efficient secondary emission surfaces with or without the quality of high photosensitiveness. It is also apparent that the electrodes 3, 4 and 5 may take on various shapes and arrangements, for example, the primary cathode may consist of a semicylinder with a secondary cathode in the form of a cylinder of smaller diameter than the uated envelope containing a primary cathode having a light sensitive surface, a light source impinging upon said surface, a secondary cathode and an anode, said electrodes extending in the same direction and the cathodes being inclined angularly with respect to one another whereby the light impinging on the primary cathode is given a multiple reflection effect between the cathodes, a source of electromotive force connected between the cathodes of such strength and polarity that the photo-electrons are impelled toward the secondary cathode with a velocity suflicient to produce secondary electrons, a source of electromotive force connected between the secondary cathode and the anode adapted to impel the secondary electrons toward the anode and an output circuit connected between the secondary cathode and the anode.

2. In combination, a tube comprising an evacuated envelope containing a primary cathode, a secondary cathode and an anode inclined angularly with respect to one another, each of said cathodes having a light-sensitive surface, a light source for activating said surface whereby photoelectrons are produced, means for accelerating the photoelectrons between the cathodes with a velocity sufiicient to produce secondary electrons, means for impelling the secondary electrons to- Ward. the anode and an output circuit connected between one of the cathodes and the anode.

3. A photoelectric tube comprising an envelope containing a pair of plate-like electrode-elements positioned in non-parallel relationship, said electrode elements being coated with light sensitive material on faces adjacent each other, and a grid-like anode element through which electrons may pass interposed between said plates.

4. A photoelectric tube comprising an envelope, a pair of lei-dimensional light sensitive members supported therein in inclined relationship relative to each other and having thereon a coating of light-sensitive material adapted to release photoelectrons when subjected to light, means includsensitive members toward another of said members with a velocity sufficient to produce secondary electrons, and electrical connections to said accelerating means and one of said light sensitive members.

6. In combination, a photoelectric tube comprising an envelope containing a pair of angularly positioned light-sensitive members, an electron permeable anode interposed therebetween,

and means for charging said electrode and one of said light-sensitive members positive with respect to the other of said light-sensitive members.

7. A phototube comprising a light sensitive photocathode, a solid plate secondary emitter electrode positioned to receive photoelectrons emitted from said photocathode, and a grid-like collector electrode surrounding said secondary emitter electrode.

8. A phototube for utilizing secondary emission to produce primary emission amplification comprising a light sensitive primary electron emitting photocathode, a secondary electron emitting cathode, and a collector electrode surrounding said secondary emitter cathode for collecting said primary and secondary electrons.

KENNETH T. BAINBRIDGE. 

